The present invention relates to a portable telephone with a built-in charger for use as a public communication apparatus.
A recent portable telephone uses a lithium battery or the like as a power source. In use of the portable telephone, the battery is discharged so that the discharged battery needs to be charged again. Recharge of the battery has been carried out by a charger provided separately from the portable telephone.
FIG. 3 shows the configuration of such a conventional portable telephone. In FIG. 3, the reference numeral 31 represents a portable telephone. A lithium-ion battery pack 33 is charged by a charger 32. A user makes the charger 32 carry out its charging operation while monitoring the respective terminal voltages of the portable telephone 31 and the lithium-ion battery pack 33 by a voltage detector 37 and controlling the charger 38 by a charge controller 39. At that time, the charge-restart is carried out when the terminal voltage of the lithium-ion battery pack 33 becomes lower than a charge-restart threshold voltage. One and the same table of the charge-restart threshold voltage is always used because the respective operation states of a calling circuit 34, a keyboard with backlight 35, a radio transceiver 36, etc. of the portable telephone 31 cannot be discriminated.
Since the charge-restart was thus carried out in the charger for the conventional portable telephone on the basis of a constant charge-restart threshold voltage regardless of the operation states of various sections, a problem was caused so that a lost of capacity of the battery after charge was generated when the load of the portable telephone was large, for example, when a charge-discharge-charge cycle was extremely short because of a terminal voltage drop at the time of speaking or backlight lighting.